My invention comprises a variable assist power steering gear system for use in automotive vehicles wherein the desired steering pressure for any given steering torque and vehicle speed is controlled instantaneously by an onboard microprocessor unit. My invention is adapted particularly to be used in a rack and pinion steering gear system of the kind disclosed in my U.S. Pat. Nos. 4,516,471; 4,485,883; 4,063,490 and 4,561,521.
In each of these prior art power steering gear mechanisms a gear rack is adjustable in the direction of its axis. The rack is connected to the steering gear linkage of a wheeled vehicle and is engaged by a drive pinion connected to a driver controlled steering shaft by a torsion bar. When the vehicle operator applies a steering torque to the steering shaft, the torsion bar deflects as driving torque is distributed to the pinion.
The pinion is connected to a valve sleeve of a rotary steering valve assembly and the steering shaft is connected to an inner valve element within the valve sleeve. Upon deflection of the torsion bar the inner valve element of the rotary valve assembly is adjusted with respect to the rotary valve sleeve. This valve adjustment controls pressure distribution to each of two fluid pressure working chambers of a fluid motor that applies a fluid pressure force to the rack to effect a power assist.
The fluid pressure pump in a steering system of this type is a positive displacement pump that has a flow control valve to provide a constant flow rate. Steering pressure is varied by controlling the effective fluid flow area through the steering valve and that area in turn is determined by the degree of deflection of the torsion bar.
In the arrangement shown by U.S. Pat. No. 4,561,521 I have provided a dual flow delivery path to the steering valve, one path being defined in part by a speed sensitive bypass valve. The effective flow area through the bypass valve is proportional to vehicle speed so that a reduced flow occurs when the vehicle is operated at high speeds and relatively high flow occurs when the vehicle is operated at low speeds or when the vehicle operator is engaged in parking maneuvers. The bypass valve is controlled by an electric actuator which in turn is controlled by a vehicle speed sensitive electronic module.